Study: source of organic matter affects Bay water quality

Each
time it rains, runoff carries an earthy tea steeped from leaf litter, crop
residue, soil, and other organic materials into the storm drains and streams
that feed Chesapeake Bay.

A
new study led by researchers at William & Mary's Virginia Institute of
Marine Science (VIMS) reveals that land use in the watersheds from which this
“dissolved organic matter” originates has important implications for Bay water
quality, with the organic carbon in runoff from urbanized or heavily farmed landscapes
more likely to persist as it is carried downstream, thus contributing energy to
fuel low-oxygen “dead zones” in coastal waters.

The
study appears in the April issue of the Journal of Geophysical Research,
and was highlighted by the journal’s publisher, the American Geophysical Union,
as an “AGU Research Spotlight” in their print and online channels.

The
study was authored by VIMS post-doctoral researcher Dr. Yuehan Lu (now at the
University of Alabama), VIMS Professor Elizabeth Canuel, Professor Jim Bauer of
Ohio State University, Associate Professor Youhei Yamashita of Hokkaido
University in Japan, Professor Randy Chambers of William &
Mary, and Professor Rudolf Jaffé of Florida International University.

Low-oxygen
dead zones are a growing problem in Chesapeake Bay and coastal ecosystems
worldwide. While most management practices focus on reducing inputs of nitrogen
and other nutrients known to fuel dead zones, Canuel says “organic matter from
the watershed may also contribute. One goal of our study was to examine the
quality of organic matter derived from streams and its potential to contribute
to dead-zone formation.”

As
streams and rivers carry dissolved organic matter downstream, bacteria or
sunlight can modify it into compounds and forms that are more difficult for
organisms to use. While the team’s research showed no significant difference in
bacterial degradation of organic matter from cleared or forested watersheds,
Canuel says it did show that “organic carbon in runoff from watersheds affected
by human activity is less susceptible to solar degradation than that from
forested watersheds.”

“Urban
organics” thus remain at higher levels longer, says Canuel, “delivering more
organic material to the river mouth and increasing the likelihood that
low-oxygen conditions will develop in downstream locations such as estuaries
and the coastal ocean.”

The
research team conducted their study using samples taken from seven small
streams that flow into the James and York rivers. Three of these streams drain
forested watersheds, with 87 to 100% tree cover, while the other four (including Powhatan Creek) drain watersheds largely
converted by human activity into pasture, cropland, or pavement and buildings.

The
authors aren’t yet sure why the organic carbon from the more developed
watersheds is less vulnerable to breakdown by sunlight in rivers and streams,
but suggest that it might be because it has already been exposed to appreciable
sunlight in the less shady urban and agricultural environment.

Says
Canuel, “Urban organics may persist downstream because their more photoreactive
compounds have already been degraded due to greater light exposure in urban
areas, farm fields, and pastures, leaving only the more photo-resistant,
refractory compounds to wash into the coastal zone.”

The
team’s findings provide one possible mechanism for an observed increase in the
concentration of dissolved organic carbon in the surface waters of North
America and Europe during the last few decades, and have implications for
management of water quality in coastal zones worldwide.

“Our results show that future studies should assess not only the
quantity of dissolved organic carbon entering our rivers and streams, but also
its source,” says Canuel.

“Understanding how organic matter from developed and
undeveloped watersheds behaves in the aquatic environment will contribute to
the development of more effective watershed management practices and hopefully
more successful efforts to reduce the number, extent, and duration of
low-oxygen dead zones.”